A typical engine fuel delivery system, including a carburetor, introduces fuel into an established flow of air created by the reciprocating action of the cylinders and synchronized valve train. With a carburetor, for example, as the liquid fuel is drawn into the air flow by the venturi design of the carburetor throat and fuel nozzle, some of the liquid fuel is immediately vaporized, while the remainder is atomized. It is the atomized fuel which produces the cloudy appearance of the air-fuel mixture downstream of the carburetor. The atomized fuel exists as liquid droplets of varying sizes suspended in the air flow. The efficiency of a particular fuel delivery system in converting liquid fuel to gaseous fuel depends upon its particular design. Ideally, any atomized fuel exists as very small sized liquid droplets, and vaporizes prior to entering the cylinder. One method utilized by the prior art to maximize the vaporization and minimize the droplet size of liquid fuel in the carburetor is the use of an injector nozzle through which some pressurized fuel is vaporized, and the remainder of the of the pressurized fuel is atomized into a relatively fine mist.
In typical naturally asperated multi-cylinder internal combustion engines, such as those used in the automotive industry, a substantially continuous flow of an air-fuel mixture is produced by a carburetor and delivered in successive charges to individual cylinders to be combusted therein. The air-fuel mixture travels through passageways formed in the intake manifold, which is adapted for receiving the air-fuel mixture from the carburetor and for delivering the air-fuel mixture to the intake port of the respective cylinders formed in the engine block. One particular arrangement of the passageways within the intake manifold includes an open or central plenum which has a plurality of individual port runners leading therefrom to respective cylinders and associated intake valves. The air-fuel mixture is drawn through the central plenum, through the port runner and into the cylinder. The port runners originate at the bottom of the central plenum and are oriented generally perpendicular to the direction of flow of the air-fuel mixture through the central plenum, so that the flow must turn about 90.degree. as it passes from the central plenum to the port runners.
The combustion of the air-fuel mixture within a given cylinder is dependent upon many factors. Two of the most important factors are the amount of fuel present in the cylinder and the phase state in which it exists. The most efficient combustion of the air-fuel mixture occurs when the fuel is present as vapor rather than as atomized liquid droplets suspended in the air. Preferably, all of the fuel in the air-fuel mixture has been vaporized prior to the initiation of combustion in the cylinder.
The presence of liquid fuel droplets in the combustion chamber reduces the power output and fuel efficiency of the engine. Liquid fuel in the combustion reduces the heat of combustion, thereby limiting the power output of the engine. Much of the fuel which is present as liquid does not combust and is exhausted unburnt from the cylinder without producing power. If fuel droplets are present in the air-fuel mixture at the time of combustion, the negative effects on combustion are minimized if the droplet size is minimized (i.e. atomization is maximized).
Engines are usually designed to operate on a uniform distribution of the air-fuel mixture to each cylinder so that each cylinder produces about the same amount of power as a result of combustion. Thus, the power output of an engine is maximized when the fuel delivery system delivers equal amounts of fuel and equal amounts of air to each cylinder under all operating conditions. However, due to physical layout and other design compromises, many engines suffer from a firing order imbalance which produces an unequal distribution of air and fuel from cylinder to cylinder. This unequal distribution of air and fuel produces a variation in the air-fuel ratio between the cylinders which is manifested as unequal amounts of liquid fuel droplets and the unequal distribution of the various sized droplets. The unequal air-fuel ratio results in some cylinders running too lean, while other cylinders run too rich. Such conditions may be determined by measuring the temperature of the exhaust gasses from each cylinder. The leaner that a cylinder operates, the higher the temperature of combustion and of the exhaust gases. Thus, in an engine with a firing order imbalance, the temperatures of the exhaust gases of the cylinders will not be equal to each other, with the leanest cylinder having the highest temperature. The exhaust gas temperatures of a typical engine with a firing order imbalance may vary by 150.degree. or more between cylinders.
In an ideal engine in which all of the fuel has been vaporized prior to reaching the port runners, a firing order imbalance would not produce such variations in the air-fuel ratio, since the air and gaseous fuel would remain relatively homogenous and flow along streamlines, independent of the operation of the engine.
Despite the objective of maximizing vaporization of the fuel at the point at which it is admixed with the air flow in the carburetor, the air-fuel mixture exiting the carburetor typically includes vaporized fuel and entrained liquid fuel droplets. These fuel droplets have masses significantly greater than the mass of the gaseous fuel molecules. The suspended liquid droplets tend to fall out of suspension from the air-fuel mixture as it travels from the carburetor to the cylinders, due at least in part to the changes in direction of the flow along the air-fuel passageway. The fuel which falls out of suspension may flow into the cylinder along the bottom of the port runners. The fuel which is present in the air-fuel mixture as vapor does not fall out of suspension.
FIGS. 1 and 2 illustrate the liquid fuel droplets falling out of suspension. FIG. 1 shows a typical prior art carburetor 10 which includes valve 12 located in passageway 14 upstream of fuel orifice 16. As previously mentioned, fuel orifice 16 may comprise a venturi jet through which liquid fuel is drawn into the air stream through passageway 14, by venturi action, or may comprise a fuel injector which atomizes and vaporizes fuel that is forced under pressure therethrough.
Valve 12 is rotated to control the flow of air through passageway 14, which concomitantly controls the flow of fuel from orifice 16. Carburetor 10 is secured to flange 18 of intake manifold 20 adjacent inlet 22, with gasket 24 interposed therebetween. Manifold 20 includes open or central plenum 26 which communicates about its lower periphery 28 with a plurality of port runners 30, as will be discussed below. Each port runner 30 communicates with a respective cylinder inlet formed in the engine block (not shown) and cylinder. Central plenum 26 communicates with port runners 30 through port opening runner 32. Thus, an air fuel passageway is formed from inlet 22, through central plenum 26 and through the respective port runner 30. This open plenum manifold 20 receives the flow of the air-fuel mixture from carburetor 10 and delivers the flow to each respective cylinder.
FIG. 2 is a schematic representation of the multitude of streamlines 34 of the flow through central plenum 26 as the flow is bent or directed at lower periphery 28 into a respective port runner 30. As is shown, streamlines 34 tend to compress, or get closer together near bottom 36 of plenum 26 as they negotiate the turn adjacent thereto, and eventually expand downstream of port runner openings 32 as shown generally at 38. Fuel in the air-fuel mixture which exists as vapor is present in the form of molecules. The low mass of the individual fuel molecules allow the vaporized fuel to flow essentially along streamlines 34, remaining in the flow as it negotiates the turn at bottom 36 of central plenum 26. The fuel vapor molecules are generally intermixed well with the air molecules, and are generally uniformly distributed throughout the air-fuel mixture flow. The air-vaporized fuel mixture is not subject to the problems of firing order imbalance, since the low mass of the air and fuel molecules allow them to respond quickly to changes in the flow as the sequential opening and closing of the intake valves occur. Schematically depicted liquid droplets 40, 42 and 44 are less likely to negotiate the change in direction of the air-fuel mixture flow as illustrated in FIG. 2, and tend to fall out of suspension due, it is believed, to their inability to travel along the curved streamlines 34, because of the droplets' inertia. The largest liquid droplets, illustrated as 40, tend to be relatively unaffected by the curved streamlines 34, particularly in the central region 46 of central plenum 26, where the streamlines tend to stagnate or disperse due to turbulence. As illustrated in FIG. 2, droplets 40 tend to travel relatively straight downwardly and impact bottom 36 at 48. Upon impact, large droplets 40 will "splatter", yielding some vaporized fuel due to the mechanics and energy of the impact, and yielding smaller liquid droplets, generally illustrated as 40a. The vaporized fuel will mix with the air-fuel mixture flow. The smaller atomized remnant droplets 40a of large liquid droplets 40 may either become entrained in the air-fuel flow, or impact bottom 36 of central plenum 26 and remain thereon.
Liquid droplets 42 are illustrated as being smaller than liquid droplets 40, and are affected to a greater degree by curved streamlines 34. These intermediate sized droplets 42 are illustrated as impacting bottom 36 at 50, producing some vaporized fuel, and some smaller droplets 42a due to the mechanics and energy of the impact, similar to that described above with respect to droplets 40.
Yet smaller droplets 44 are illustrated as being affected even more by curved streamlines 34, but eventually striking bottom 36 at point of impact 52, yielding vaporized fuel and yet smaller liquid droplets 44a in accordance with the description above.
Although large droplets 40 are illustrated in the central region 46, and small droplets 44 near the wall of central plenum 26, it will be understood that the droplet size is not necessarily a function of the droplet location. Small droplets will occur in the central region 46, while large droplets will occur near the wall. Small droplets in the central region 46 will tend to follow the streamlines 34, while large droplets 40 near the wall will tend to impact bottom 36.
Whether a particular liquid fuel droplet remains entrained in the air flow as it negotiates turns in the air-fuel passageway, particularly at the bottom of the central plenum, depends upon several factors. Some of these factors are droplet size, the flow rate and speed of the air-fuel mixture and the location of the fuel droplet relative to the center of the central plenum. For example, very small fuel droplets flowing downwardly through the central region of the central plenum may remain entrained in the air-fuel flow, while medium size droplets flowing near the outer periphery of the central plenum may fall out of suspension. Also, it is believed that as liquid droplets cross steamlines, there is a tendency for them to break up into smaller droplets, with some of the resulting droplets being redirected by the flow and remaining entrained in the air-fuel flow. Additionally, the amount of turbulence created varies with the physical parameters of the central plenum, firing order, and flow velocity, and can affect the degree of atomization and degree of vaporization of the liquid droplets. Transient conditions which result from changes in the flow rate, which may be chaotic in nature, can have an impact on the amount of liquid droplets which negotiate the flow path bends.
Liquid droplets which impact bottom 36 may leave some residual liquid fuel thereon. The accumulation of this liquid fuel, if not vaporized or reentrained by the flow adjacent bottom 36, may produce a stream of liquid flowing along the bottoms of port runners 30, and into the cylinders. The presence of this liquid further reduces the total energy of combustion of that particular cylinder.
The tendency of the liquid fuel droplets to fall out of suspension due to directional changes in the air-fuel flow and to cross the flow streamlines contributes to or enhances the affects of the firing order imbalance. FIG. 3 shows a schematic representation of intake manifold 20 with central plenum 26 and eight port runners 30. As can be seen, FIG. 3 shows port runners 30 and associated port runner openings 32 as being uniformly distributed about the lower periphery 28 of central plenum 26. However, as shown in FIG. 4, pairs of port runners 30 may be grouped together, having a common port runner opening 32a, or having immediately adjacent respective port runner openings 32 disposed about lower periphery 28 of central plenum 26. The actual physical location of the port runners, along with their particular length and characteristics, when coupled with a given firing order, tend to set up a flow resonance which favors the flow of the air-fuel mixture towards a particular group of cylinders, i.e., through a particular group of port runners. It is believed that this resonance can impart directional momentum to the entrained fuel droplets toward the "favored" port runners. This results in the non-uniform distribution of fuel between cylinders. While the air and vaporized fuel flowing into and through central plenum 26 is believed to be generally uniformly distributed to the port runners despite the firing order imbalance, there is a significant variation in the air-fuel ratio of the flow to each respective cylinder. It is believed that this results due to the tendency of liquid fuel droplets to come out of suspension, and the affect that the firing order imbalance has on directing these liquid fuel droplets toward the "favored" cylinders. While the air and vaporized fuel molecules have masses low enough to allow them to respond quickly to the changes in direction of flow which occurs in manifold 20 due to the sequential opening and closing of the intake valves, the liquid droplets cannot respond as quickly due to their momentum.
Firing order imbalance, and the flow resonance created thereby, is dependent upon the operating conditions of the engine, such as engine speed, load, ambient conditions, fuel, etc. For example, at a given engine speed, certain cylinders will be "favored", tending to have a richer air-fuel ratio than the other cylinders. Correspondingly, certain other cylinders will have a lean air-fuel ratio. The resultant of the variation in the air-fuel ratio between the cylinders is a variation in the efficiency and the power between the cylinders. Some cylinders have less vaporized fuel and more atomized fuel then others. The droplet sizes and distribution of the various sized droplets varies from cylinder to cylinder, affecting the completeness and efficiency of the combustion in the respective cylinder.
As shown in FIG. 5A, certain cylinders in this example tend to burn hotter than other cylinders at a particular engine speed due to firing order imbalance and the accompanying flow resonance. This indicates the variation in the air-fuel ratio between cylinders. The leaner the mixture, the higher the temperature of combustion and resultant gas temperatures. The richer the mixture, the lower the temperature of combustion and exhaust gas temperatures. FIG. 5A shows certain cylinders having hotter exhaust gasses than other cylinders. FIG. 5B illustrates a shift in the flow resonance due to a change in the engine speed. FIGS. 5a and 5B illustrate the differences in firing order imbalance under fixed operating conditions (other than engine speed) occurring at different engine speeds.
The temperature of the exhaust gases is not only reflective of the fuel ratio variation, but also is dependent upon the degree of homogenous mixing of the air and fuel, as well as the quantity and size of liquid fuel droplets entrained in the air-fuel mixture. As is well known in the art, the larger the droplets, the less efficiently the fuel is combusted. This is due to the amount of free oxygen molecules which are able to surround the fuel droplet.
Thus, there is the need in the art to alleviate this problem.